1. Field of the Invention
The present invention relates to a screen printing machine and a printing unit that print solder on a circuit board on which electronic components are to be mounted.
2. Description of the Related Art
A screen printing machine is used to print solder on lands (circuit electrode portions) of a circuit board. Normally, one screen printing machine has a single printing position. Therefore, one screen printing machine can process only one circuit board at a time. With this in view, Japanese Patent Application Publication No. JP-A-2000-168040 and Japanese Patent Application Publication No. JP-A-2005-262689 each disclose a screen printing machine that can process two circuit boards at a time.
In recent years, production lines for circuit boards (a production line in which supply of circuit boards, printing of solder onto the circuit boards, and mounting of electronic components onto the circuit boards are performed sequentially) tend to have dual lanes. The screen printing machine according to each of JP-A-2000-168040 and JP-A-2005-262689 is used for a single-lane production line, in which only printing of solder onto circuit boards is performed on two circuit boards at a time. Thus, it is difficult for the screen printing machine according to each of JP-A-2000-168040 and JP-A-2005-262689 to be used for dual-lane production lines.
In dual-lane production lines, it is necessary to use two screen printing machines. FIG. 26 is a schematic view of a production line in which two screen printing machines are arranged in series (hereinafter occasionally referred to as “series production line”). FIG. 27 is a schematic view of a production line in which two screen printing machines are arranged in parallel (hereinafter occasionally referred to as “parallel production line”). In FIGS. 26 and 27, corresponding members are denoted by the same reference symbols.
The series production line is first described. As shown in FIG. 26, production lines 105f and 105r include a board supply device 100, a first screen printing machine 101, a second screen printing machine 102, and an electronic component mounting machine 103. The board supply device 100, the first screen printing machine 101, the second screen printing machine 102, and the electronic component mounting machine 103 are arranged in line along a transfer direction for circuit boards in which circuit boards are transferred (a direction from the left to the right).
The first screen printing machine 101 includes two board transfer sections 101f and 101r arranged side by side in the front-rear direction. Of the board transfer sections 101f and 101r, only the board transfer section 101f on the front side has a printing position 109f. The second screen printing machine 102 includes two board transfer sections 102f and 102r arranged side by side in the front-rear direction. Of the board transfer sections 102f and 102r, only the board transfer section 102r on the rear side has a printing position 109r. 
For a circuit board 106f transferred through the production line 105f, solder is printed at the printing position 109f of the first screen printing machine 101. For a circuit board 106r transferred through the production line 105r, solder is printed at the printing position 109r of the second screen printing machine 102.
The parallel production line is then described. As shown in FIG. 27, production lines 105f and 105r include a board supply device 100, a first screen printing machine 101, a second screen printing machine 102, and an electronic component mounting machine 103. The first screen printing machine 101 and the second screen printing machine 102 are disposed side by side in the front-rear direction.
The first screen printing machine 101 includes a single board transfer section 110f. The board transfer section 110f has a printing position 111f. The second screen printing machine 102 includes a single board transfer section 110r. The board transfer section 110r has a printing position 111r. 
For a circuit board 106f transferred through the production line 105f, solder is printed at the printing position 111f of the first screen printing machine 101. For a circuit board 106r transferred through the production line 105r, solder is printed at the printing position 111r of the second screen printing machine 102.
Conventionally, as described above, two screen printing machines (the first screen printing machine 101 and the second screen printing machine 102) are used for the dual-lane production lines 105f and 105r. 
In the case where the first screen printing machine 101 and the second screen printing machine 102 are disposed in series as shown in FIG. 26, however, the total length of the production lines 105f and 105r in the left-right direction (the total dimension of the production lines 105f and 105r in the transfer direction for the circuit boards 106f and 106r) becomes accordingly long. Therefore, the production lines 105f and 105r occupy a large installation area on a floor of a plant. That is, the space efficiency is reduced. Meanwhile, in the case where the first screen printing machine 101 and the second screen printing machine 102 are disposed in parallel as shown in FIG. 27, the width of the production lines 105f and 105r in the front-rear direction (the dimension of the production lines 105f and 105r in a direction generally perpendicular to the transfer direction for the circuit boards 106f and 106r) becomes accordingly large. This also reduces the space efficiency. As described above, the use of two screen printing machines reduces the space efficiency. Also, portions of the lanes at which the screen printing machines forming the production lines are disposed cannot be straight because of the widths of the two screen printing machines (the dimensions of the two screen printing machines in the front-rear direction).
During a tooling change for the circuit boards 106f and 106r (a change in type of circuit boards to be produced), it is necessary to replace a replacement part such as a back-up plate, a back-up block or a back-up pin, a screen mask, and a squeegee in accordance with the type of the circuit boards 106f and 106r for each of the production lines 105f and 105r. 
The production lines 105f and 105r are arranged side by side in the front-rear direction irrespective of whether the first screen printing machine 101 and the second screen printing machine 102 are arranged in series (FIG. 26) or in parallel (FIG. 27). That is, in either case, the two printing positions 109f and 109r (FIG. 26), or the two printing positions 111f and 111r (FIG. 27), are arranged side by side in the front-rear direction.
A housing of the first screen printing machine 101 is normally provided with a replacement access port that opens in a Y direction (toward an operation space 107f) for replacement of a replacement part. Likewise, a housing of the second screen printing machine 102 is normally provided with a replacement access port that opens in a Y direction (toward an operation space 107r) for replacement of a replacement part. Thus, in order to smoothly replace a replacement part in the production line 105f on the front side, it is preferable that an operator 108f approaches the replacement access port from the front side. Meanwhile, for replacement in the production line 105r on the rear side, it is preferable that an operator 108r approaches the replacement access port from the rear side.
Therefore, for smooth replacement of a replacement part, two operation spaces 107f and 107r are necessary on the front and rear sides of the production lines 105f and 105r, respectively, as shown in FIGS. 26 and 27. That is, large operation spaces 107f and 107r are necessary. Also from this point of view, the use of two screen printing machines reduces the space efficiency.
In order to avoid reduction in space efficiency, it is conceivable to provide only one of the operation spaces 107f and 107r. In the case where only one of the operation spaces 107f and 107r is provided, however, it is difficult for the operators 108f and 108r to replace a replacement part in a farther one of the production lines 105f and 105r. Specifically, it is difficult for the operator 108f staying on the front side of the production line 105f to replace a replacement part disposed in the production line 105r on the rear side. Meanwhile, it is difficult for the operator 108r staying on the rear side of the production line 105r to replace a replacement part disposed in the production line 105f on the front side.
Thus, if only the operation space 107f, of the operation spaces 107f and 107r, is provided, it is necessary to stop the production line 105f for convenience of operation when the operator 108f staying on the front side of the production line 105f replaces a replacement part in the production line 105r on the rear side. That is, it is necessary to stop not only the production line 105r but also the production line 105f which does not need replacement of a replacement part in the first place. This reduces the production efficiency for the circuit boards 106f and 106r. Likewise, if only the operation space 107r, of the operation spaces 107f and 107r, is provided, it is necessary to stop the production line 105r when the operator 108r staying on the rear side of the production line 105r replaces a replacement part in the production line 105f on the front side. This reduces the production efficiency for the circuit boards 106f and 106r. 